A Single Spin Switches the Steady-State Phase of an Open Quantum System
Abstract
Changing a many-body system by one constituent is normally expected to produce only a vanishing correction to an intensive observable. Here we show that, above a critical dissipation imbalance, adding or removing one spin at fixed intensive controls switches the steady state of a dissipative collective spin between a phase-averaged ring and a south-polar fixed point. Unlike previous one-spin sensing through dark-state interference, parity here determines whether the Dicke ladder samples an interior zero of the nonlinear jump amplitude. The integer-spin ladder samples this zero, making Dicke states with m<0 transient, whereas the half-integer ladder misses it and remains a single recurrent class. Exact finite-size steady states show that exponential competition between stationary weights amplifies this microscopic connectivity difference, yielding a first-order dissipative phase transition confined to the odd-N sequence. Multisector one-spin loading and state-unresolved removal confirm the switch, while the exponentially increasing switching time reflects the metastable isolation of the competing macroscopic basins. Our results establish representation-lattice sampling as a route to phase control, suggesting parity-based detection of single-spin addition or removal.
Related
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BibTeX
@misc{jie2026single,
title = {{A Single Spin Switches the Steady-State Phase of an Open Quantum System}},
author = {Jianwen Jie},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2608.28390},
eprint = {2608.28390},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.28390},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - A Single Spin Switches the Steady-State Phase of an Open Quantum System
AU - Jianwen Jie
PY - 2026
DA - 2026/09/08
PB - arXiv
DO - 10.48550/arxiv.2608.28390
UR - https://arxiv.org/abs/2608.28390
AB - Changing a many-body system by one constituent is normally expected to produce only a vanishing correction to an intensive observable. Here we show that, above a critical dissipation imbalance, adding or removing one spin at fixed intensive controls switches the steady state of a dissipative collective spin between a phase-averaged ring and a south-polar fixed point. Unlike previous one-spin sensing through dark-state interference, parity here determines whether the Dicke ladder samples an interior zero of the nonlinear jump amplitude. The integer-spin ladder samples this zero, making Dicke states with m<0 transient, whereas the half-integer ladder misses it and remains a single recurrent class. Exact finite-size steady states show that exponential competition between stationary weights amplifies this microscopic connectivity difference, yielding a first-order dissipative phase transition confined to the odd-N sequence. Multisector one-spin loading and state-unresolved removal confirm the switch, while the exponentially increasing switching time reflects the metastable isolation of the competing macroscopic basins. Our results establish representation-lattice sampling as a route to phase control, suggesting parity-based detection of single-spin addition or removal.
ER -